U.S. patent number 5,216,529 [Application Number 07/824,785] was granted by the patent office on 1993-06-01 for holographic code division multiple access.
This patent grant is currently assigned to Bell Communications Research, Inc.. Invention is credited to Eung G. Paek, Jawad A. Salehi.
United States Patent |
5,216,529 |
Paek , et al. |
June 1, 1993 |
Holographic code division multiple access
Abstract
A technique for all-optical Code Division Multiple Access (CDMA)
system based on optical holography is disclosed. In this technique
the energy of an incoming information light signal is spread over a
spatial domain by a two-dimensional spatial encoder which includes
a mask having regions of first and second transmission
characteristics corresponding to the unique code assigned to a
particular source. Subsequent decoding, which is accomplished by an
optical matched filter through the use of a hologram, spatially
despreads the energy of the information light signal and produces a
focused light beam which serves as input to a code division
detector.
Inventors: |
Paek; Eung G. (Freehold,
NJ), Salehi; Jawad A. (Bedminster, NJ) |
Assignee: |
Bell Communications Research,
Inc. (Livingston, NJ)
|
Family
ID: |
25242310 |
Appl.
No.: |
07/824,785 |
Filed: |
January 15, 1992 |
Current U.S.
Class: |
359/29; 359/559;
382/210; 382/280; 708/816; 708/821 |
Current CPC
Class: |
G03H
1/041 (20130101); G03H 1/16 (20130101); H04J
14/005 (20130101); G03H 1/0005 (20130101); G03H
2001/0066 (20130101) |
Current International
Class: |
G03H
1/16 (20060101); G03H 1/04 (20060101); H04J
14/00 (20060101); G03H 001/16 () |
Field of
Search: |
;359/11,29,559,560,561
;382/31,42 ;364/822,827 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Psuedo-Random Sequences and Arrays", F. J. MacWilliams et al.,
Proceedings of the IEEE, vol. 64, No. 12, pp. 1715-1729, Dec. 1976.
.
"Signal Detection by Complex Spatial Filtering", A. Vander Lugt,
IEEE Trans. of Info. Theory, IT 10:2, pp. 139-145, Apr. 1964. .
"Introduction to Fourier Optics", J. W. Goodman, McGraw-Hill Book
Company, pp. 171-177, 1968. .
"Emerging Optical Code-Division Multiple Access Communications
Systems", J. A. Salehi, IEEE Network Magazine, vol. 3, No. 2, pp.
31-39, Mar. 1989. .
"Spread Spectrum for Commercial Communications", D. L. Schilling et
al., IEEE Comm. Magazine, vol. 29, No. 4, pp. 66-79, Apr. 1991.
.
"Spread-Time Code Division Multiple Access", P. Crespo et al.,
Globecom '91, pp. 836-840, 1991..
|
Primary Examiner: Arnold; Bruce Y.
Assistant Examiner: Ryan; J. P.
Attorney, Agent or Firm: Suchyta; Leonard Charles Peoples;
John T.
Claims
What is claimed is:
1. A code division multiple access system for interconnecting a
plurality of sources to a corresponding plurality of receivers,
each of the sources including an optical interface to produce an
optical source signal, the system comprising
a plurality of optical encoders for generating an associated
plurality of encoded optical signals, each of said optical encoders
being responsive to the optical source signal from a corresponding
one of the sources, with each of said optical encoders including a
unique spatial optical encoding mask for generating a corresponding
one of said encoded optical signals, said mask having an optical
transmission pattern determined from a preselected set of patterns
wherein said patterns correspond to sequences with predetermined
correlation properties,
an optical broadcast network, coupled to said optical encoders, for
combining said encoded optical signals to generate a composite
optical signal as an output of said broadcast network, and
a plurality of optical detectors, responsive to said composite
signal, for generating an associated plurality of decoded optical
signals, each of said decoded optical signals being assigned to one
of the receivers, each of said optical detectors being arranged to
detect a corresponding one of said encoded optical signals in said
composite optical signal, and wherein each of said optical
detectors includes a holographic decoder corresponding to said
unique spatial optical encoding mask used to generate said
corresponding one of said encoded optical signals, said holographic
decoder producing said each of said decoded optical signals.
2. The system as recited in claim 1 wherein each of the sources
includes an information signal and a light source for generating
the optical source signal as representative of the information
signal, wherein each said optical encoder includes a collimating
lens for optically spreading the optical source signal to produce a
spatially spread light signal, and wherein said encoding mask
includes transparent and opaque regions arranged according to one
of the patterns in said set of patterns, and said encoding mask is
positioned to modulate said spread light signal to thereby generate
said corresponding one of said encoded signals.
3. The system as recited in claim 1 wherein each of the sources
includes an information signal and a light source for generating
the optical source signal as representative of the information
signal, wherein each said optical encoder includes a collimating
lens for optically spreading the optical source signal to produce a
spatially spread light signal, and wherein said encoding mask
includes first regions of transmission having a first phase shift
and second regions of transmission having a second phase shift,
said first and second regions arranged according to one of the
patterns in said set of patterns, and said encoding mask is
positioned to modulate said spread light signal to thereby generate
said corresponding one of said encoded signals.
4. The system as recited in claim 1 wherein each of said
holographic decoders comprises
a Fourier Transform lens for filtering said composite signal to
produce a transformed optical signal,
a hologram corresponding to said unique spatial optical encoding
mask, said hologram being located at the focal distance from said
Fourier Transform lens and serving as an optical matched filter to
convert said transformed optical signal to a holographic optical
signal, and
a focusing lens aligned on the matched filter axis of said hologram
to convert said holographic optical signal to said each of said
decoded optical signals.
5. The system as recited in claim 1 wherein the sources and the
receivers are geographically disperse, and wherein the system
further comprises
a first plurality of optical fibers interconnecting the plurality
of sources to said plurality of optical encoders, and
a second plurality of optical fibers interconnecting said plurality
of optical decoders to the plurality of receivers.
6. A system for interconnecting a plurality of incoming
information-bearing light signals to a corresponding plurality of
outgoing information-bearing light signals, the system
comprising
an plurality of encoders, responsive to the incoming light signals,
for generating a corresponding plurality of modulated light
signals, wherein each of said encoders is responsive to a
corresponding one of the incoming light signals and includes:
an input collimating lens for spatially spreading said one of said
incoming light signals to produce a spread light signal; and
an encoding mask, optically coupled to said incoming collimating
lens, for modulating said spread light signal to produce a
corresponding one of said modulated light signals, said mask
including first regions having a first transmission characteristic
and second regions having a second transmission characteristic
wherein the arrangement of said first regions and second regions on
said mask correspond to a preselected pattern from a set of code
patterns,
an optical broadcast network for combining said modulated light
signals into a composite light signal,
a plurality of decoders, coupled to said broadcast network, for
generating the outgoing optical signals, wherein each of said
decoders is assigned to a corresponding one of said encoders and
includes:
a Fourier Transform lens for receiving said composite signal to
produce a transformed light signal;
a hologram, located at the optical focal distance from said Fourier
Transform lens, for generating a holographic light signal from said
transformed light signal, said hologram representative of one of
said code patterns assigned to said corresponding one of said
encoders; and
an outgoing focusing lens, aligned on the matched filter axis of
said hologram, for generating a corresponding one of the outgoing
light signals from said holographic light signal.
7. A method for combined encoding and decoding of an incoming
optical signal to produce a outgoing optical signal comprising the
steps of
spreading the incoming optical signal with a collimating lens to
produce a spread optical signal,
modulating said spread optical signal with a unique spatial optical
encoding mask to produce an encoded optical signal, said mask
having an optical transmission pattern determined from a
preselected set of patterns wherein said patterns correspond to
sequences with predetermined correlation properties,
filtering said encoded optical signal with a Fourier Transform lens
to produce a transformed optical signal,
modulating said transformed optical signal with a hologram
positioned at the optical distance from said Fourier Transform lens
to produce a holographic light signal, said hologram being
representative of said encoding mask, and
filtering said holographic light signal with a focusing lens to
produce the outgoing optical signal, said focusing lens being
aligned on the matched filter axis of said hologram.
Description
FIELD OF THE INVENTION
This invention relates generally to optical fiber communication
systems and, in particular, to code division multiple access
systems wherein the information content communicated between each
source/receiver pair is decoupled from the encoding/decoding
operations.
BACKGROUND OF THE INVENTION
The proliferation of fiber-optic cables and the ever increasing
demand for new broadband services is moving future
telecommunication networks toward all-optical networks. By design,
all-optical networks perform key signal processing such as
switching, multiplexing, demultiplexing, amplification, and
correlation, with optical systems and avoid electrical-to-optical
and optical-to-electrical conversions. Optical systems or optical
signal processing should alleviate the predicted bottleneck that
could occur with complex high-speed electronic switches,
multiplexers, demultiplexers, and so forth, because all-optical
techniques are potentially much faster than electrical signal
processing. Several new classes of optical networks are emerging.
In particular, code division multiple access (CDMA) networks using
optical signal processing technique have been recently introduced.
For example, the special issue on "Optical Multiaccess", as
published in the IEEE Network Magazine, vol. 3, no. 2, March 1989
provides an overview of this emerging field.
In a typical CDMA system, multiaccess is achieved by assigning
different, minimally interfering code sequences to different user
pairs. Users then communicate by imprinting their message bits upon
their own unique code, which they transmit asynchronously (with
respect to the other transmitters) over a common channel. A matched
filter at the receiver end ensures that message bits are detected
only when they are imprinted on the proper code sequence. This
approach to multiaccess allows transmission without delay and
handles multiaccess interference as an integral part of the
scheme.
Processing gain (PG) for CDMA techniques is a critical parameter
which may be used to judge the relative merits of CDMA systems.
Processing gain is broadly given by the ratio of total transmitted
bandwidth to information bandwidth of a transmitter. The value of
PG establishes an upper bound on the number of users/transmitters
that can be simultaneously active on a given CDMA system. Presently
known CDMA techniques such as spread-spectrum and spread-time
(which will be discussed in detail below) can only incrementally
increase the PG since the total transmitted bandwidth is usually
fixed, implying that the information bandwidth must be decreased in
order to increase the PG. A large reduction in information
bandwidth is difficult to achieve for arbitrary information
sources.
In addition, with conventional CDMA techniques all transmitters
have essentially the same signal format and data rate. This
precludes a mixed multiuser environment wherein it is desired to
transmit analog voice, low rate data, a TV signal, and so forth
simultaneously over the CDMA system.
SUMMARY OF THE INVENTION
These shortcomings and other limitations are obviated, in
accordance with the present invention, by arranging a CDMA system
such that: parameters determining the processing gain are decoupled
from both total bandwidth and information bandwidth; and the signal
format and information rate of each user is independent of other
users.
Broadly, the CDMA system in accordance with the present invention
interconnects numerous sources to corresponding receivers through
an optical broadcast network. Optical encoders are interposed
between the sources and the optical broadcast network, and optical
detectors are located between the optical broadcast network and the
receivers. Each optical encoder includes a two-dimensional, spatial
encoding mask for encoding a light signal produced by the
associated source. The spatial encoding mask is determined from
sequences having appropriate autocorrelation and crosscorrelation
properties, so that each encoder generates a corresponding
optically encoded signal. The optical broadcast network combines
the numerous optically encoded signals produced by the encoders to
generate a composite optical signal composed of all the encoded
light signals. Each optical detector is assigned to detect one (or
more) of the optically encoded signals, that is, the information
content of one of the sources, and each optical detector includes a
holographic decoding mask corresponding to the assigned source.
The organization and operation of this invention will be understood
from a consideration of the detailed description of the
illustrative embodiment, which follows, when taken in conjunction
with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a diagram of the all-optical spread-space code division
multiple access system in accordance with the present
invention;
FIG. 2 illustrates two holographic masks assigned to first and
second source-receiver pairs;
FIG. 3 lists the array of (0,1) pixels from an exemplary
holographic mask;
FIG. 4 depicts an illustrative embodiment of an optical broadcast
network used to form a composite light signal;
FIG. 5 is an arrangement for generating unique holograms used in
optical matched filters; and
FIG. 6 illustrates the placement of collimating lens relative to
the hologram of each decoding arrangement to achieve matched filter
detection.
DETAILED DESCRIPTION
To properly place in perspective the inventive aspects of the
present invention, an overview of the present invention couched in
terms related to prior art techniques is first presented. This
approach also has the advantage of introducing notation and
terminology. Next, a detailed description of an illustrative
embodiment completes the disclosure.
Overview
The techniques of spread-spectrum and spread-time CDMA spread the
energy of the information signal over a wide frequency band or over
a long time period, respectively. For an elucidating discussion of
spread-spectrum techniques, the article entitled "Spread Spectrum
for Commercial Communications," by Schilling et al, as published in
IEEE Communications Magazine, Vol. 29, No. 4, April 1991 is
particularly relevant. Also, the paper entitled "Spread Time Code
Division Multiple Access" by Crespo, Honig, and Salehi, as
published in the Globecom Proceedings, December, 1991 provides a
detailed discussion of the spread-time approach.
In accordance with the present invention, which is referred to
generically as spread-space CDMA, energy of the information signal
is spread over a large spatial domain. In general, spreading the
energy of a given information signal and the subsequent despreading
of the energy is known as encoding and decoding of the information
signal. Therefore, in the spread-spectrum technique, encoding and
decoding are obtained in time domain, and in the spread-time
technique, encoding and decoding are obtained in frequency domain,
whereas in the spread-space technique, encoding and decoding are
obtained in spatial domain.
In both spread-spectrum and spread-time CDMA techniques,
information waveforms (modulating signals) are represented as
digital signals, i.e., they exclude the use of analog signals, and
all users have identical bit rate and signal format. However, in
spread-space CDMA technique, the modulating signal for each user
can take on any form (digital or analog), any rate, and any shape.
For example, in a multiuser environment a particular user can be
sending analog video while other users are sending digitized voice,
analog voice, low rate data, and a very high rate data signal. This
means that the spread-space CDMA technique remains transparent to
the form of modulation format of each user. This advantage is
obtained by transferring the CDMA encoding and decoding to spatial
domain while modulating the information signal in the time
domain.
Processing Gain (PG) for spread-spectrum and spread-time systems is
defined as the ratio of total transmitted bandwidth (basically, a
function of encoding and decoding speed) to the information
bandwidth. Processing Gain is the single most important design
parameter in any CDMA system. Its value puts a limit on the number
of users that can operate simultaneously in a CDMA system. To
increase the number of users in a CDMA system from its present
value (operating at some bit-error rate), the PG for that system
must increase. There are two ways to increase the PG. First, by
increasing the channel bandwidth (that is, the encoding and
decoding speed), or second, by reducing the information rate. In
optical networks where channel bandwidth is not as scarce a
commodity as in other systems, one may design all-optical encoders
and decoders that have speeds which are 3 to 4 orders of magnitude
faster than the information source. But, in CDMA systems such as in
radio cellular telephony channel bandwidth is finite and scarce.
Thus, reducing the information rate may prove to be the only
realistic alternative. The difficulty in increasing PG, thereby
increasing the number of simultaneous users, with the
spread-spectrum and spread-time CDMA techniques is due to their
dependency on the input information rate. However, the uncoupling
between the two domains of signal processing, i.e., spatial domain
for CDMA encoding and decoding and time domain for information
modulation, contributes to another and a very important feature in
spread-space CDMA, namely, the uncoupling of the PG from its input
information rate. That is, input information rate for each user can
be different while the PG for all the users is the same. Processing
Gain in an optical spread-space CDMA, which will be discussed in
more detail below, is proportional to the number of pixels in a
mask and the value of PG is independent of input information rate
or from any modulation format. The number of distinct pixels in a
mask of area A can be as high as ##EQU1## where .lambda. is the
wavelength of the light used in the system. For A=1 cm.sup.2 and
typical operating wavelengths (1-2 microns) this corresponds to
10.sup.6 -10.sup.8 pixels or PG for spread-space CDMA system. Since
the PG for typical spread-spectrum or spread-time CDMA techniques
is 10.sup.2 -10.sup.3, then spread-space CDMA can potentially
support 4 to 5 orders of magnitude more usres. For example, if one
to ten percent of PG is taken as the number of users that can be
supported by any of the CDMA techniques, then spread-space CDMA can
support as many as tens of thousands to a few million users where
each user can utilize any input information rate and any modulation
format.
Illustrative Embodiment
Spread-space CDMA system 100, depicted partly in block diagram form
and partially in component form in FIG. 1, interconnects sources
101, 102, . . . 103 to receivers 111, 112 . . . 113, respectively.
(System 100 may also be referred to as a Holographic CDMA system
for reasons that will become apparent as the description proceeds.)
Each source 101, 102, or 103 can produce either analog or digital
signals, may operate at an arbitrary information rate, and need not
be compatible with the other sources. Moreover, the information
generator included within each source (not explicitly shown), such
as a voice signal or a TV camera, may be electronic so that each
source 101, 102, or 103 would include an electro-optical interface
to its corresponding fiber medium. Each receiver 111, 112, or 113,
which is matched to a corresponding source in the sense that each
receiver is arranged to detect the analog or digital format at the
incoming information rate, either electronically or
electro-optically depending on the original information generator
at the corresponding source.
Since each source 101, 102, or 103 is arranged with an interface so
as to propagate an optical signal representative of the information
content of the source, a monochromatic light signal is propagated
onto a fiber optic medium associated with each source; for example,
source 101 launches a monochromatic light signal onto fiber 121.
Holographic encoding for the monochromatic light signal emanating
from fiber 121 is obtained by: (1) collimating the monochromatic
light signal with collimating lens 141; and (2) modulating the
collimated monochromatic light signal emerging from lens 141 with a
two-dimensional mask 151 having an array of two-dimensional code
elements, that is, modulation is obtained by placing mask 151
behind collimating lens 141. Mask 151 has a transmission
characteristic which is proportional to a two-dimensional code.
Exemplary code elements are members the set (0,1), where a 0
corresponds to opaque area on mask 151 and a 1 corresponds to a
transparent area on mask 151. (Another exemplary set is (+1,-1),
where +1 corresponds to transmission with zero phase shift, and -1
corresponds to transmission with a .pi. phase shift). Two typical
two-dimensional codes, designated s.sub.1 (x,y) and s.sub.2 (x,y),
where x and y are spatial coordinates, are shown in FIG. 2, and
illustratively correspond to masks 151 and 152, respectively. For
each exemplary mask 151 or 152, there are 1024 (32.times.32)
pixels, that is, the code length for each mask is 1024. The
32.times.32 pixels array for mask 151 is listed in FIG. 3. It is
possible to have as many as 10.sup.6 -10.sup.8 pixels in a 1 cm by
1 cm mask.
The two-dimensional codes for Holographic CDMA can be obtained from
binary sequences of length n, whose autocorrelation is either 1 or
##EQU2## by the conventional method discussed in the paper
"Pseudorandom Sequences and Arrays", authored by F. Macwilliams and
N. Sloane, and published in the Proceedings of the IEEE, Vol. 64,
No. 12, pp. 1715-1729, December, 1976. The two-dimensional codes as
described in the reference generally satisfy the requirements of
randomness and have autocorrelation and crosscorrelation properties
that are necessary for the family of two-dimensional codes used for
Holographic CDMA systems. For a pseudorandom array (a
two-dimensional code with flat autocorrelation function) with n
pixels there are n different arrays, with each array obtained
simply by considering each shift of the original array to be a
different array. Then for a Holographic CDMA system with K users,
where K.ltoreq.n, each shift can be assigned to a different
source/user in system 100.
The light signals transmitted through masks 151-153 in FIG. 1,
designated as S.sub.1 (x,y), S.sub.2 (x,y), and S.sub.K (x,y),
respectively, serve as inputs to optical broadcast network 105.
Network 105 is arranged to form a composite signal, designated
S.sub.T (x,y), which has the following form: ##EQU3## where K is
the number of sources/users. Thus S.sub.T (x,y) is a linear
combination of all the modulated light signals transmitted by masks
151-153.
The arrangement of FIG. 4 depicts an illustrative embodiment for
optical broadcast network 105 of FIG. 1. Optical signals S.sub.1,
S.sub.2, S.sub.i, and S.sub.K (the argument (x,y) for each signal
has been dropped for ease of presentation), serve as inputs to
network 105. S.sub.1 is reflected from mirror 410 onto beam spitter
420. S.sub.2 also impinges on beam splitter 420 so that the output
from splitter 420 in the downward direction towards beam splitter
421 may be expressed as (S.sub.1 +S.sub.2)/2. S.sub.i, that is, the
signal originating from the i.sup.th source (not shown explicity in
FIG. 1) and impinging on network 105, passes through attenuator 431
and excites beam splitter 421 in the horizontal direction. The
attenuator is set to 0.5 so that the signal emanating from splitter
421 is the downward direction towards beam splitter 422 is
expressed as (S.sub.1 +S.sub.2 +S.sub.i)/4. Finally, S.sub.K is
passed through attenuator 432, with its attenuation value set at
0.25, and impinges on beam splitter 422 along with the output of
splitter 421. The composite signal emerging from splitter 422 in
the horizontal direction, which may be represented by (S.sub.1
+S.sub.2 +S.sub.i +S.sub.K)/8, is passed through optical gain
device 441. If device 441 has a gain of 8.0, then the signal
emerging from device 441 is S.sub.T as defined in equation (1).
Again with reference to FIG. 1, composite signal S.sub.T (x,y)
emerges on K optical paths from network 105. The first output
optical path feeds S.sub.T (x,y) to Fourier Transform lens 161.
Hologram 171, also labeled as S.sub.1 Hologram in FIG. 1, is placed
at the focal length distance (F.sub.L) behind lens 161. The signal
transmitted through hologram 171 is intercepted by focusing lens
181 placed in a strategically located position behind hologram 171;
the precise placement will be discussed below shortly. Lens 181
delivers a demodulated optical signal to fiber 131, and in turn,
fiber 131 propagates this demodulated optical signal to receiver
111. The combined operation of the cascade of Fourier Transform
lens 161, hologram 171, and focusing lens 181 is referred to as
optical holographic CDMA decoding using an optical matched
filter.
Holographic CDMA decoding is obtained by arranging lens 161,
hologram 171 and lens 181 to implement the optical matched filter;
this filter maximizes the ratio of peak signal energy to rms noise.
One realization of this matched filter is obtained by an optical
method introduced by A. Vander Lugt in the article entitled "Signal
Detection by Complex Spatial Filtering", as published in the IEEE
Transactions of Information Theory, IT 10:2, pp. 139,145, April,
1964. The optical matched filter has a transfer function which is
the complex conjugate of the code image spectrum.
With reference to FIG. 5, there is shown hologram generator
arrangement 200 for generating each S.sub.i Hologram for the
s.sub.i (x,y) mask, i=1,2, . . . K, of FIG. 1. Arrangement 200 uses
reference beam 241 to interfere with the output of Fourier
transform lens 260 at hologram 270. Hologram 270 is any medium that
registers light intensity, such as photographic film. Laser source
210, which is illustratively an argon laser operating at 514.5 nm,
illuminates collimating lens 220; in turn, the output of lens 220
is directed to beam splitter 230, with the horizontally transmitted
component impinging on mirror 240 and the vertically deflected beam
being modulated by mask 250 representative of array s.sub.i (x,y),
i=1, 2, . . . , or K. The angle of light signal 241 reflected by
mirror 240 is .alpha.. The output light from mask 250 impinges on
Fourier Transform lens 260. Finally, both the light signal from
lens 260 and the reflected light from mirror 240, shown as beam
241, are focused on hologram 270. Arrangement 200 creates the
desired intensity pattern on hologram 270 so that when each
hologram representative of each unique s.sub.i (x,y) mask is
embedded in system 100 of FIG. 1, matched filter detection may be
effected.
Again with reference to FIG. 5, if F.sub.1 (p,q) denotes the output
of lens 260, which displays a light signal which is the Fourier
transform of s.sub.1 (x,y) at its back focal plane, with p and q
representing spatial frequency, and if R(p,q) represents the light
coming from mirror 240, with
R(p,q)=.vertline.R(p,q).vertline.e.sup.j.phi.(p,q), where
.vertline.R(p,q).vertline. is a constant and .phi.(p,q) is linear
in phase, then the intensity pattern on the holographic recording
film is, ##EQU4## The fourth term in equation (2) represents the
desired filter function, F.sub.i * (p,q), multiplied by the linear
phase factor of R(p,q) since .vertline.R(p,q).vertline. is
constant. Once the matched filters, that is, the holograms, for
different codes are sequentially produced beforehand by hologram
generator 200, the holograms are then physically located at the
receiving end of system 100, namely, as holograms 171, 172,
173.
The exact placement of, for example, focusing lens 181 relative to
hologram 171 in FIG. 1 is depicted in detail in FIG. 6. It can be
demonstrated that the first two terms from equation (2) give rise
to a light beam aligned with optical axis 172 of FIG. 6. For
purposes of this invention, this light signal is ignored. Another
transmitted light beam emerges from hologram along optical axis 173
which is offset from optical axis 172 by the downwardly directed
angle .alpha.. This light signal along axis 173 is the output from
the optical matched filter and corresponds to the fourth term in
equation (2). Finally, for completeness, the third term in equation
(2) corresponds to the beam emerging from hologram 171 along
optical axis 174 at an upward angle .alpha., and this beam is also
ignored. Complete details for aligning lens 181 with hologram 171
may be found in the text "Introduction to Fourier Optics", authored
by J. W. Goodman, published by McGraw-Hill Book Company, 1968;
particular reference should be made to pages 171-177.
Briefly, by way of an operational description, the component
S.sub.1 (x,y) present in S.sub.T (x,y) will have a wavefront
curvature which will be brought into focus by Fourier Transform
lens 181 to thereby generate a bright intensity light signal
focused at the input to fiber medium 131; this focusing occurs
since S.sub.1 hologram 171 is matched to mask 151, that is, the
s.sub.1 (x,y) mask.
On the other hand, when, for example, light component S.sub.2 (x,y)
in the composite signal S.sub.T (x,y) is incident on the hologram
171, the output will have a random-like wavefront curvature which
will not be brought to a bright focus by the Fourier Transform lens
181. If it is assumed the properly decoded signal has a bright spot
with intensity one, any other signal present in the composite
signal will have, on average, an intensity ##EQU5## where NM=n is
the number of pixels in a mask (code) with N.times.M dimensions.
The large contrast in the intensities between a matched, decoded
signal and an unmatched, decoded signal is used to distinguish
between correctly and incorrectly addressed signals, that is, to
distinguish among sources.
It is to be understood that the above-described embodiments are
simply illustrative of the application of the principles in
accordance with the present invention. Other embodiments may be
readily devised by those skilled in the art which may embody the
principles in spirit and scope. Thus, it is to be further
understood that the methodology and concomitant circuitry described
herein is not limited to the specific forms shown by way of
illustration, but may assume other embodiments limited by the scope
of the appended claims.
* * * * *